Symbiosis Between Diatoms and Cyanobacterial Colonies

Species-rich diatom communities were found to occur within colonies of the nitrogen-fixing cyanobacterium Rivularia atra Roth in the brackish Baltic Sea. These communities have never been reported before. To understand how the special conditions within the cyanobacterial colonies (limited light, presence of host mucilage, chemical excretions by the host) affected diatom community composition, comparisons were made with diatom communities epiphytic on macroalgae sampled from the same stones as the Rivularia colonies. Motile diatoms of the genera Amphora, Berkeleya, Cymbella, Entomoneis, Epithemia, Lunella, Mastogloia, Nitzschia and Rhopalodia dominated the Rivularia-associated communities whereas attached diatoms of the genera Achnanthes, Cocconeis, Diatoma, Fragilaria, Gomphonema, Hyalosira, Licmophora, Rhoicosphenia and Tabularia dominated the epiphytic communities on macroalgae. Diatom species richness, Shannon diversity and evenness were all significantly higher in the Rivularia communities. The diatoms probably profit from the symbiosis with Rivularia in several ways: (1) by protection against grazing (2) by protection against physical disturbances, (3) by using cyanobacterial mucilage as a substratum for motility and (4) by excretion of inorganic and organic nutrients from Rivularia cells. It is more difficult to identify advantages for Rivularia in this symbiosis, but nutritional advantages may exist for the cyanobacterium as well.